1. Field of the Invention
This invention relates to a method for removing solvent from a thermoplastic resin. More particularly to a continuous process for recovering a thermoplastic resin from a solution of said resin in a solvent comprising introducing said solution into a first vessel through atomizing means along with steam; removing solvent vapor and water vapor overhead resulting in a bottom product water slurry of polymer-solvent granules maintained at about 1 to 50 weight percent solids with makeup hot water; removing said bottom product slurry from said first vessel, removing hot water from said slurry, resulting in polymer granules with absorbed solvent and water; introducing said granules along with superheated steam into a second vessel wherein remaining solvent and water are volatilized from said granules while being carried through said second vessel by said superheated steam; separating polymer granules from the steam-solvent vapor mixture using separating means; recycling said steam-solvent vapor mixture for injection to the first vessel, for steam jacket heating of the first vessel or for preheating the polymer solution before injection into the first vessel; and recovering granular polymer product from said separating means.
2. Description of the Prior Art
One of the methods suggested in the past for the separation of polycarbonates and in general of polymers from their solution in organic solvents is removing solvent by feeding a concentrated solution of polycarbonate resin into one or more heated apparatuses, in particular into an extruder comprising degassing zones under vacuum for volatilizing the solvents. Said method involves rather high operation costs and high capital investment for the special equipment required.
Another method for the above process is disclosed in U.S. Pat. No. 3,772,262 which uses a "spray drying" technique, consisting of atomizing the solution of polymer-organic solvent in a stream of hot inert gas or gases, so as to facilitate the removal of the solvent. Such methods require the use of equipment having a low specific productivity and the use of large volumes of heated gases e.g., steam, which because of its high solvent content, cannot be effectively reused in other spray drying stages. In addition, only a small amount of recoverable energy contained in said steam can be recycled for preheating the solvent-polymer solution feed before the temperature of the solution approaches the condensing temperature of the steam-solvent mixture. This method also produces a solid polymer product with a low apparent density.
In addition, previous patents such as U.S. Pat. Nos. 3,031,438, 3,684,783 and 3,590,026 disclose methods, termed slurry granulation methods, wherein a polymer-hot organic solvent solution is dispersed in water and the solvent is removed or stripped in one or more stages of slurry granulation wherein steam heats the dispersion to supply latent heat of evaporation and also carries away solvent vapor. Multiple stage slurry granulation methods generally utilize higher temperatures in subsequent stages for increased solvent removal. The drawbacks of these methods are high consumption of energy, due to the necessity of using multiple stages in order to achieve a substantial removal of the solvent and above all, the fact that the product contains considerable quantities of water which must subsequently be removed by means of long and expensive drying processes wherein all of the energy in this large volume of relatively low temperature steam can not be utilized since only a limited amount of energy can be recovered in recycling for use in preheating the solvent-polymer solution or for heating the slurry since the high low-boiling solvent content of the steam mixture and its relatively low temperature render it unsuitable for use elsewhere in the process, e.g. steam stripping in earlier stages. Also, the low boiling, relatively non-condensable solvent vapor insulates heat exchanger surfaces from the high boiling steam. A common disadvantage inherent in the prior art is the lack of adequate heat or energy recovery or heat recycling capabilities at a time of high energy costs.